The present invention relates to a sheet holding apparatus that sequentially stacks sheets transported from a printer or a copier or other image forming apparatus for holding, or that sequentially stacks sheets such as original sheets that are supplied to a scanner, facsimile or other image reading apparatus, and more particularly to a sheet transport apparatus that employs a sheet stacking amount detection method for detecting the maximum amount of sheets that can be stacked on a tray, and a sheet holding apparatus.
Generally, a sheet holding apparatus that holds sheets which have been formed with images by an image forming apparatus, such as a printer, in a holding tray, or sheets which have been read by an image reading apparatus, such as a scanner, and that sequentially transports sheets from a transport path into a holding tray for stacking and holding is widely known in the art.
Subsequently fed sheets can become jammed inside the apparatus on such a sheet holding apparatus when a number of sheets that exceeds a predetermined amount is held in its tray. Furthermore, sheets that have already been stacked on that tray can be pushed out causing them to become scattered. Therefore, it is necessary to apply some measure to stop the apparatus on the upstream side by judging, such as by using detection means, whether the sheets that have been stacked on the tray have reached a maximum limit of the tray.
Conventionally, as a means for detecting the maximum stacking amount of a tray, sheets that are sequentially transported are counted at the upstream side of the tray. When this counted value has reached a predetermined number of sheets, it is determined that the maximum amount that can be stacked on a tray has been reached and the apparatus is stopped. An example of this method is disclosed in the Japanese Patent Publication (KOKAI) 6-247617. Another type of detection means is disclosed in the Japanese Patent Publication (KOKAI) No. 7-172684. Here, a tray is provided with a detection lever which touches the uppermost sheet of those sheets stacked on the tray. A photoelectric sensor is used to detect the position of this detection lever so that when a predetermined level of the sheet surface is reached, the system judges that the maximum amount of sheets has been reached.
In the method described above for counting sheets, a sensor, such as a photodiode, is disposed to detect the presence of a sheet in the sheet transport path. In one well-known approach to this method, a counter counts the number sheets that pass this sensor; and in another well-known approach, as described in the Japanese patent mentioned above, a counter counts the number of sheet feeding command signals to ascertain the number of sheets that have been fed.
In the latter method, which uses a detection lever and sensor to detect the level of the stacked sheets, a lever member that is swingably supported is arranged to hang downward toward the top of the tray from thereabove. The leading edge of the lever detects the height level of the sheets by touching the uppermost sheet on the stack. A photosensor, such as a photodiode, is arranged on the base of the lever. When the detection lever has detected a predetermined height level, a photosensor detects that position thereby determining that the sheets have reached a maximum holding level. Such system is widely known.
On the other hand, in a sheet holding apparatus incorporated with a sheet transport apparatus such as a copier, or scanner for which compactness is a requirement, a tray has a capacity to hold several tens of sheets. Such an apparatus must hold a variety of paper thicknesses from very thin to thick sheets. However, depending on the transport conditions of the sheets, large anomalies can occur in the detection of the maximum volume of sheets that can be stacked. This can cause the apparatus to be stopped without being completely filled to the maximum amount that the tray can actually hold. This causes an operator to feel less secure with the apparatus and it can also cause paper jams or sheets to be disturbed in the tray.
Therefore, in an apparatus particularly configured with a compact holding tray to enable a more compact apparatus overall, it is necessary to detect the maximum amount of sheets in the tray as accurately as is possible. At the same time, normally such apparatuses have a plurality of operating modes whose transport speeds differ according to the processing conditions. For example, reading speeds can differ for reading color images and for reading black and white images. Thus, it is preferable to be able to vary the maximum number of sheets that can be stacked in accordance with the differences in these operating modes.
However, the maximum amount of sheets that can be stacked can vary, depending on the thickness of the sheets in use. Therefore, normally, the maximum amount of sheets that can be stacked is set to a standard. This is particularly true for sheets that are the maximum thickness, and that are held when they have the largest curl if the method described above for counting sheets is used. Therefore, regardless of whether using thick or thin sheets, and sheets being held with the least amount of curl, and whether there is still room left in the tray to accommodate more sheets, the apparatus still must be stopped if an error occurs. Furthermore, if previous sheets remain on the stacking tray, there is the potential for an overflow because there are no means for detecting such sheets. This can cause serious trouble on the machine such as damaging the original sheets by becoming jammed inside the machine.
On the other hand, with the method of detection that uses a detection lever to detect the level of the sheets, the leading edge of a sheet pushes this detection lever upward each time a sheet is transported and is stacked on the stacking tray. Therefore, the detection sensor must be able to detect the position of the detection lever at a position with even the slightest interval between sheets to judge that the number of sheets has reached a predetermined maximum level. Therefore, this system can experience detection errors when transporting sheets at high speeds or with short intervals therebetween.
Of particular note, when the volume of sheets on the stacking tray is low, the amount of movement of the detection lever is greater, so the sensor that detects this can judge sheet levels comparatively more accurately. However, as the number of sheets approaches the tolerance level of the tray, the amount of detection lever movement decreases thereby inviting erroneous detection of the level of sheets.
Furthermore, the number of misdetections of the position of a detection lever increases as the sheet transport speed increases, or as the intervals between sheets become shorter. This is because the leading edge of the next sheet pushes the detection lever upward and out of the way before the detection lever has had a chance to exit a sensor. This position (with the detection lever seemingly in a continuously raised position) is mistakenly judged as the surface level of the sheets.
In view of the problems associated with accurately judging the maximum amount of sheets that can be stacked (sequentially) in a stacking tray, an object of the present invention is to provide a sheet stacking amount detection method and sheet holding apparatus that can accurately judge the maximum amount of sheets that can be stacked on a stacking tray despite sheets already existing on the stacking tray, or if sheets are excessively curled, and that can accurately judge the maximum amount of sheets that can be stacked on a stacking tray even when conditions for transport are different, such as different transport speeds for the sheets, or the different intervals between sheets (caused by differences in operating modes).
Further objects and advantages of the invention will be apparent from the following description of the invention.